Abstract: Tannins are secondary metabolites produced by higher plants. Tannins and tannin-linked organic nitrogen complexes play an important role in the nitrogen cycling at the plant-soil interface. The mechanisms by which tannins participate in the nitrogen cycling mainly include complexing organic nitrogen, affecting soil microbial activities and soil enzyme activities. Previous studies have not explored the complexation capacity of tannins to various organic nitrogen compounds, and the degradation mechanism of mycorrhizal fungi and saprophytic fungi on tannin-linked organic nitrogen complexes. Therefore, this review focuses on the complexation capacity of tannins to bind with various organic nitrogen compounds, the degradation mechanism of the complexes, the inhibition of soil enzyme activities by tannins and the influences of tannins on activities of soil microorganisms. Roles such as slowing of litter decomposition, inhibition of net nitrogen mineralization, and effects of tannins on net nitrification and nitrogen fixation in the nitrogen cycle is summarized. The findings of this review indicate that tannins can complex most of the organic nitrogen compounds; the structure and concentration of the tannins present can significantly affect the inhibitory actions of tannins on soil enzyme activities and net nitrogen mineralization, and soil microbial activity and diversity. These conclusions lay a foundation for further understanding of the role of tannins in nitrogen cycling at the plant-soil interface.

Key words: tannins, tannin-organic nitrogen complexes, nitrogen cycling, soil enzyme activity